Monday, October 15, 2007

Seal stomachs I: Harps and Hoods

The start of this post is here

We steamed slowly through loose pack ice. Harps and hoods are seals of the Arctic pack, and we were sailing through the roof of their world. Kjell and Bjørn shot seals that were resting on floes.

All seals haul themselves out of the water at some time in their lives. Generally, hauled-out equals on-land. But for many polar seals, ashore is ice, simply a change in the phase of water. They live in and on the ocean, resting by climbing onto a handy, frozen chunk of it.

Our imagery of harps comes from pupping time, when mothers-to-be congregate in their thousands. The pictures create an impression of gregariousness. But pupping lasts only a few weeks. For the rest of the year, harps circuit the Arctic Atlantic, and just how sociable they are is not clear. We had stumbled into some harps, clumped enough to suggest sociability. It was late winter, the time for laying down fat, preparing for the stresses of pupping and mating in a few short weeks. Were there so many seals because there was plenty of food nearby, or because harps tend to hang out together? Another unknown.

Flecked in amongst the harps were some hoods.

Hooded seals seem to lead a lonelier - or at least more solitary - life. Even when they give birth, females keep to themselves. One male wins the right to accompany her while she nurses her pup. And hooded seals nurse their young for the shortest span of any mammal – four days. So a hooded seal “family” (as they're known) mum, pup and male – is a rather brief relationship.

It's through their pups that we make our links between harps and hoods. We find pups of both species on pack ice, close to the ice edge, together. More or less. “We” being, almost always, hunters. Most of the interest that people have in harps and hoods is thanks to the fur wrapped around pups. Harp seal pups are no longer killed for their famous white coats, but for the fur that reveals itself once that white moults - a couple of weeks' difference. Hooded seal babies are killed for the fur they're born with – the two-tone that gives them one of their names - bluebacks. They, too have a white coat, but they moult it before they're even born. Only four days suckling with mum - they have no time to waste.

Harps and hoods differ in other ways too. Harps are the smaller, with females growing to about five-and-a-half feet and somewhere under 300 pounds, and males marginally bigger. Hoods are much larger again, with females of nearly 7 feet and 350 pounds. But male hoods are giants in comparison, reaching over eight feet and weighing in at around 650 pounds. Adult male hoods also have a huge, flubby nose, complete with a large reddish balloon that they can blow out of their left nostril. For reasons that are, no doubt, obvious to other hooded seals.

Unlike harps, there's no population of hooded seals breeding off northwestern Russia. Hoods are seals of the western Atlantic basin: three of four subpopulations live among the ice and islands between Canada and Greenland. The seals we were killing were probably from the westernmost subpopulation of hoods, breeding in the West Ice (off eastern Greenland, the name comes from it being west to Norwegians) – the same place as the central population of harp seals.

Although hoods don't occur in the Barents Sea, individuals occasionally find their way to the Caribbean. Harp seals ignore the lure of the tropics.

It's not only in two dimensions - latitude and longitude - that harps and hoods differ. There's also the ocean's third dimension – depth. Harps are fairly shallow divers, usually staying within a couple of hundred metres of the surface. Hoods, on the other hand, are much deeper divers, and can swim down to over a kilometre.

How do we know where these seals dive to, in remote Arctic waters? Nuclear submarines on the lookout? No. We know this because scientists (Garry among them) have attached satellite-linked time-depth recorders (acronymed into SLTDRs) to these seals. SLTDRs are miniature computers (they range in size from a little bigger than a TV remote control to about the size of four bars of soap) that are glued onto the top of seals' heads. There they sit, sensors recording time and depth every few seconds, storing and assimilating numbers. Then, when the right satellite appears in the sky, and the seal – and hence the antenna – is at the surface, the SLTDR dutifully dumps its data via radio link.

From the way the message is received by the satellite, the position of the transmitter (and hence, the seal) can be calculated. So we can infer, from the depths and shapes of dives (coupled with their changes in geographical position), what seals have been doing underwater – travelling, feeding, sleeping. And then from looking at where and how deep seals are on their feeding dives, we can make inferences as to what seals are eating. Because the transmitter is glued onto the hair on a seal's back, it just falls off when the seal moults, as they do every year.

But we weren't doing anything as technically sophisticated as attaching computers to seals. On our expedition, we were just killing, and cutting open. It's the oldest, and simplest, scientific method available to infer marine mammals' diet, but it's still used in the 21st Century. And we weren't really where we'd intended to be when we'd started the cruise a couple of weeks earlier. A minor mishap had brought us south, to Iceland, from the West Ice a few hundred sea miles north.

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